Organohalosilanes have found many uses in the chemical arts. By way of illustration, certain organohalosilanes, such as trimethylchlorosilane, are useful as silylating agents, polymer end-blockers and capping reagents in organic and pharmaceutical synthesis. Several processes for the preparation of organohalosilanes have been proposed. For example, methylchlorosilanes can be produced by the "direct reaction" of methyl chloride with a silicon-copper mass (see, e.g. U.S. Pat. No. 2,380,995).
The direct reaction produces a mixture of organochlorosilanes (e.g. dimethyldichlorosilane and trimethylchlorosilane). Usually more dimethyldichlorosilane and less trimethylchlorosilane are produced by the direct reaction than are required commercially.
Redistribution (e.g., disproportionation) methods are also known in the art for the preparation of organohalosilanes, particularly for converting excess dimethyldichlorosilane to needed trimethylchlorosilane. Some of these methods utilize aluminum catalysts and reactants, such as AlCl.sub.3, and other aluminum-containing materials. A silane redistribution reaction is a rearrangement of at least two different substituents which are attached to a silicon atom or atoms. One or more silanes having at least two substituents, such as CH.sub.3 or Cl, redistribute when these substituents exchange sites of bonding to silicon. The resulting silane or silanes still have a total of four substituents or atoms attached to the silicon atom, but in ratios different from that of the starting silane or silanes. A typical silane redistribution reaction can be illustrated by the following equation: ##STR1##
A silane disproportionation reaction is a type of silane redistribution reaction that occurs when a single silane yields two or more dissimilar silanes having substituents (i.e., CH.sub.3 Cl) in differing proportions from that of the initial starting silane. A silane disproportionation reaction can be illustrated by the following equation: ##STR2##
This position of thermodynamic equilibrium in the disproportionation of diorganodihalosilanes typically favors a high concentration of the diorganodihalosilane. For example, with dimethyldichlorosilane, the following equilibrium concentrations are observed at 350.degree. C. with 2 weight percent AlCl.sub.3 : ##STR3## (see Zemany et al, J. Amer. Chem. Soc., vol. 70, p. 4222 (1948); Golosova et al, Russ. J. Phys. Chem., vol 45, p. 460 (1971) Engl. Trans.). Disproportionation of diorganodihalosilanes without any catalyst or with the catalysts described by the prior art does not provide a practical commercial synthesis of triorganohalosilanes because equilibrium is attained too slowly or not at all. Consequently, a practical synthesis of triorganohalosilanes by the disproportionation of diorganodihalosilanes requires the use of a catalyst capable of accelerating the attainment of equilibrium. The use of SiH compounds and arylsilanes as co-catalysts with AlCl.sub.3 for this purpose is known (See U.S. Pat. Nos. 2,786,861 and 3,793,357).
U.S. Pat. No. 2,717,257 describes the use of alkali metal halogen-aluminates of the general formula, MAlX.sub.4, wherein M is an alkali metal and X is a halogen, as catalysts for the redistribution of organohalosilanes.
Morozov et al (Russ. J. Gen. Chem. vol. 41, p. 1275 (1971); vol. 39, p. 2234 (1969) Engl. Trans.) report that the disproportionation of dichlorodimethylsilane is facilitated by the addition of NaAlCl.sub.4 to a fixed bed of silicon-copper contact mass in the presence of active methyl chloride molecules on the catalyst surface.
U.S. Pat. No. 2,647,136 states that aluminum chloride was the only catalyst found to accelerate the redistribution of methylchlorosilanes and that the other common Lewis acid catalysts, such as BCl.sub.3, ZnCl.sub.2, FeCl.sub.3 and CuCl, had no perceptible effect on the course of the redistribution reaction.
The use of organoaluminum compounds as catalysts in methylchlorosilane redistribution and disproportionation reactions, in general, often results in pyrophoric, potentially explosive, and readily hydrolyzable organoaluminum by-products which must be disposed via lixiviation with water or alkali. This can be extremely hazardous because of the attendant hydrolysis of methylchlorosilanes to yield hydrogen chloride. Aluminum trichloride has appreciable solubility in methylchlorosilane mixtures which can produce separation and purification problems. Plugging problems in transport lines and distillation columns are usually concomitant with its use. When aluminum trichloride is utilized in conjunction with certain co-catalysts described in the prior art, there is the possibility of product contamination or loss of catalyst by volatization.
Complexes comprising copper halides and aluminum halides are known (see, e.g., Schlapfer, et al, Inorganic Chem., vol. 17, No. 6, p. 1623 (1978); and U.S. Pat. Nos. 3,651,159; 3,647,843; and 3,592,865). These complexes have been utilized in various processes, e.g. in reactions involving the alkylation or halogenation of hydrocarbons (see U.S. Pat. Nos. 3,846,503; 3,846,504; 3,420,908; and 3,935,288). However, prior to the present invention, complexes of aluminum halide and copper halide were not utilized in reactions involving the redistribution of organohalosilanes.
Accordingly, it is an object of the present invention to provide a novel process for the preparation of organohalosilanes utilizing copper halide-aluminum halide complex catalysts.
In particular, it is an object of the present invention to provide a novel process for the redistribution (e.g., disproportionation) of organohalosilanes utilizing a copper halide-aluminum halide complex catalyst.
More particularly, it is an object of this invention to provide a novel process for the preparation of triorganohalosilanes by the catalytic disproportionation of diorganodihalosilanes utilizing a copper halide-aluminum halide complex catalyst.
A further object of this invention is to provide a process for the redistribution of organohalosilanes which does not produce hazardous by-products, involve soluble catalysts, produce product contamination or lose catalyst through volatization.